The present invention relates to a cogeneration system and more particularly to a cogeneration system employing a Stirling cycle engine for driving an electric generator wherein the "waste" heat is used to provide space heating or hot water service.
Cogeneration is a process whereby a consumer in need of either hot water and/or heat for a residence or a small business, instead of merely burning fuel to produce heat, can burn fuel to drive an electric generator and utilize the cycle heat and the waste heat from the drive means, generator and exhaust to produce the needed hot water or hot air. The electricity generated can be used on site with any surplus electric energy directed to the utility's electric grid and sold to the utility. Numerous efficiencies can be achieved with such a cogeneration system.
The second law of thermodynamics states that the quality of energy can change only in one direction, energy losses its capacity to do useful work, ultimately reaching the point of zero usefulness. As available work is consumed, the quality of energy is degraded, however, the quantity of energy remains the same. Hence, good energy saving practice strives to harness energy of the highest quality possible, that is to avoid unwanted degradation.
Following the second law of thermodynamics, it is wasteful to burn fuels just to obtain low quality energy needed for low temperature process heat such as heating a residence or a small business or producing hot water. However, by cogeneration of electricity and heat, the system efficiency can be greatly increased. By way of example, a process of producing both electricity and heat independently of one another may result in combined efficiency of only 52%. When a cogeneration approach is used, the total efficiency can increase to as high as 85%. In another example, it is possible to produce high pressure steam at a temperature of 500.degree. C. for a back pressure turbine where it is converted to mechanical energy that drives an electric generator. The steam at the outlet of the turbine is at a temperature of 150.degree. C. to 175.degree. C. and can be used to fulfill many thermal needs. The efficiency of this process is greater than the efficiency of a process used to produce equal amounts of electricity and process heat independent of one another.
Between 1980 and 1983, approximately 9,000 MW of cogeneration capacity was installed in the United States. Most of these installations are over 200 kW in size. Studies project the feasibility of a total 68,000 MW more of cogeneration capacity which is the equivalent output of approximately seventy power plants. These relative large industrial and commercial cogeneration applications are being implemented in large part due to the fact that available engines can be economically utilized and the market can be met through existing engineering technology and the large, relatively steady electric loads of the industrial or commercial applications make the economics of cogeneration favorable.
However, the majority of buildings in the U.S. are residential and light commercial, with an estimated aggregate electric load of about 140,000 MW on an annual average. To date, there has been limited commercial success in developing small scale cogeneration equipment. This results from the difficulty of economically implementing such applications using currently available equipment. The difficulties are both technical and economic in nature and include the fact that small building loads are highly variable compared to industrial cogeneration applications. This is particularly true in single family homes where fluctuations from 0.1 to 0.5 kW of electric energy occur in seconds and thermal loads are weather dependent resulting in seasonal variations. These widely varying loads complicate the task of determining the design, size, and operational strategies for systems having high duty cycles and high overall electrical and heat recovery efficiency required for economical viability. Furthermore, most engines in the 1-30 kW size currently available are designed for only 500-2,000 hours of life before complete overhaul or replacement. By contrast, small scale cogeneration requires maintenance free engines with a life at least ten times longer. Most of the cost of maintenance is associated with routine servicing which, in small sizes, tends to be dominated by high labor costs.
However, there are certain advantages to small units which might overcome many of these barriers. For example, standardized modular packages could eliminate the site specific engineering and system design cost often associated with the larger applications. The potential market in number of units is much larger for small modules than for custom engineered larger systems. This could result in economics of manufacturing scale which could overcome inherent economics of size in individual components. Many applications above 30 kW might be better served by using multiples of smaller modules rather than a single larger unit in order to reduce vulnerability to increased utility demand charges since it is unlikely that more than one unit would go down at the same time, i.e. only a fraction of generating capacity will be lost with the loss of a single module.
Current small cogeneration systems of 10 to 30 kW have been tried with a reciprocating internal combustion engine without great success. Such a system has many drawbacks. These engines have a relative short life. The maintenance costs of the engines are high and time consuming. These include lubricating oil changes, spark plug changes, etc. Reciprocating internal combustion engines are noisy and also produce vibration. The engine exhaust is high in polluting emissions and half of the waste heat is in the exhaust which results in corrosion problems due to the nitrous oxides in the exhaust.
The Stirling cycle engine, however, is well suited for cogeneration applications. This is due in part to qualities of the Stirling engine such as quiet running, primary heat rejection by cooling water, long life, low emissions, and low maintenance.
Stirling engines may be powered directly by any source of heat such as from solar energy sources, combusted gases, liquid fuels, solid fuels etc. The preferred type of Stirling engine for use in small cogeneration systems incorporates multiple gas combustors that are integrated into the structure of the engine to provide a compact and efficient energy conversion machine. This system eliminates the requirement of a separate heat pipe or other heat transport systems for transferring heat from a remote source. Individual combustors are provided for each cylinder of a multiple cylinder Stirling engine.
The output shaft of the Stirling engine is coupled to an electric generator to drive the generator. In the preferred embodiment, the generator is enclosed in the pressure hull of the engine reducing the complexity of the drive shaft coupling and seals. The electric power generated is consumed on the premises with any surplus electric energy being fed to the electric grid and sold to the local utility. The engine and generator are cooled by cooling water. The water is further heated in a condensing heat exchanger by the hot engine exhaust gases. The hot water is then used for space heating or to provide hot water service to the building. For space heating, the water can be fed to hot water radiators or used to heat air for forced air space heating.
In other embodiments disclosed below, an air conditioner can be included with the cogeneration system with the electricity produced used to power the air conditioning compressor. When operating the air conditioner, since space heat is not needed, the cooling water is routed to a radiator outside of the building for rejection of waste heat from the Stirling engine and generator.
In addition to providing more efficient use of fuels, a cogeneration system also reduces CO.sub.2 emissions. Coal combustion, often used for producing electric energy at a power plant, produces much more CO.sub.2 than natural gas combustion that is most often burned for residential and light commercial building heating purposes. Thus, electricity produced by cogeneration will reduce the amount of CO.sub.2 emissions. Furthermore, since the electricity can be used at or near its site of production, transmission line losses can be reduced. This reduces the amount of electric power which must be generated, thereby reducing the amount of fuel burned and emissions produced.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.